Process of etching metal with ammonium persulfate with recovery and recycling

ABSTRACT

PROCESS FOR ETCHING COPPER WITH AN AQUEOUS PERSULFATE SOLUTION, IN WHICH THE ETCHING SOLUTION IS REPEATEDLY REACTIVATED OVER AN EXTENDED PERIOD OF TIME BY REPEATED REMOVAL OF METAL SULFATE VALUES AND AMMONIUM SULFATE AND IN WHICH SUBSTANTIAL HYDROLYSIS OF THE PERSULFATE OCCURS DURING SAID EXTENDED PERIOD. A COMPOUND YIELDING AN ORTHOPHOSPHATE ION IS ADDED TO THE COMPOSITION OF THE ETCHING SOLUTION DURING THIS PERIOD TO MAINTAIN THE TOTAL PHOSPHATE ION CONCENTRATION AT 0.1 TO 0.35 M/1. AND AT A PH OF 1.2 TO 1.8.

"United States Patent PROCESS OF ETCHING METAL WITH AMMO- NIUM PERSULFATE WITH RECOVERY AND RECYCLING John S. C. Chiang, Cranbury, and Bernard J. Hogya, Sayreville, N.J., assignors to FMC Corporation, New York, N.Y. No Drawing. Filed Dec. 4, 1970, Ser. No. 95,398

Int. Cl. C23f 1/00; H05k 3/00 vs. Cl. 156-19 7 Claims ABSTRACT OF THE DISCLOSURE Process for etching copper with an aqueous persulfate solution, in which the etching solution is repeatedly reactivated over an extended period of time by repeated removal of metal sulfate values and ammonium sulfate and in which substantial hydrolysis of the persulfate occurs during said extended period. A compound yielding an orthophosphate ion is added to the composition of the etching solution during this period to maintain the total phosphate ion concentration at 0.1 to 0.35 M/l. and at a pH of 1.2 to 1.8.

This invention relates to the process for etching with an ammonium persulfate etching solution. In the process the used or spent etching solution is repeatedly reactivated (by removal of metal sulfate values and ammonium sulfate values) and reused.

A process of this type is described in the patent to Caropreso, Radimer and Hogya, 3,399,090 of Aug. 27, 1968, which also states that the etching solution may contain additives such as sulfuric acid or phosphoric acid in addition to ammonium persulfate. That process has been used commercially for the etching of such materials as copper circuit boards carrying a resist.

The continued use of the process, however, has resulted in some severe problems. It was found that the ammonium persulfate hydrolyzes slowly but consistently, forming ammonium bisulfate at high enough concentrations to cause a reduction of etch rate and incomplete etching of copper The presence of high concentrations of a bisulfate in the process also leads to partial dissolution of tin from the circuit boards carrying tin-lead solder or tin as the etching resist. The tin may be electroplated tin from regular acid tin baths or bright tin from the newly available bright acid tin baths. This partial dissolution of tin causes a tin build-up in the etching solution, and leaves the metal resist vulnerable for copper deposition. The copper deposition results in darkening of the metal resist which may cause interference with subsequent soldering operation. (See, for instance the etching with ammonium persulfate mentioned in the article by Missel Shelf Life Improvement of Electroplated Solder in Metal Finishing, June 1970 pages 85-89). The use of additives such as sulfuric acid or phosphoric acid in the process does not improve the situation, but worsens it. Thus, such additions of an acid to the processhas been found to accelerate the rate of the ammonium persulfate hydrolysis causing rapid ammonium bisulfate buildup in the process, presumably because the hydrolysis of ammonium persulfate is catalyzed by the acid.

To control the bisulfate concentration it has been proposed that there be added a basic copper compound such as copper hydroxide, copper carbonate or copper oxide in limited amounts such that the concentration of ammonium bisulfate is kept relatively low, e.g. between 0.15 to 0.3 M/l., which corresponds to a pH range of 0.86 to 0.64 for the etching solution. It is essential to maintain this amount of ammonium bisulfate in the process when etching circuit boards carrying tin-lead solder or tin as an etching resist. In the absence of ammonium bisulfate, the reaction products resulting from the attack of the ammonium persulfate on the metal resist are not dissolved and run onto the adjacent copper being etched. thereby producing either nonuniform or incomplete etching.

Basic copper compounds are preferred for ammonium bisulfate concentration control, because they react with the ammonium bisulfate to form equal molar amounts of copper sulfate and ammonium sulfate; thereby no major foreign compounds will be introduced to the process. However, the use of basic copper compound to control the ammonium bisulfate concentration in the process is a preventive measure and is costly. Commonly available basic copper compounds invariably contain small amounts of metallic impurities. Thus the continued addition of excessive amounts of basic copper compounds may cause impurities to build up in the etching solution which interfere with the etching reactions.

It therefore is an object of our invention to provide an improved process which has a reduced bisulfate build-up rate, and at the same time enables the etching solution to produce high quality etched circuit boards, especially those circuit boards carrying tinlead solder or tin (tin with white finish or with bright and specular finish) as an etching resist.

In accordance with one aspect of the present invention it is found that when the process is modified so as to maintain the concentration of the components of the etching solution within a limited critical range, defined below, outstanding results are obtained. Bright tin-resisted circuit boards are very cleanly etched, with the bright tin conductors retaining their original brightness. Solder-resisted circuit boards are also excellently etched, with the solder conductors remaining unmottled. Very little under cutting of the plated resist occurs, resulting in excellent etch factors. The etched circuit boards have very good shelf life. The etching solution exhibits a much lower rate of hydrolysis of the persulfate and consequent lower buildup of bisulfate (e.g. about one half of the rate obtained before the process is modified in accordance with this invention); furthermore, the etching solution has good stability in general.

In the practice of the invention a compound yielding an orthophosphate ion is added to the aqueous etching solution so that the total phosphate content is no less than 0.1 M/l. but no more than 0.35 m/l. and the pH is 1.2 to 1.8. A pH above 1.8 can result in precipitation of copper phosphate in the etcher causing a cloudly solution and reduced etch rate. In most practical operations, the concentration of ammonium persulfate in the etching solution will generally be in the range of about 0.4 to 1.3 M/l. and the concentration of dissolved copper will generally be no more than about 0.8 M/l., while the titratable hydrogen ion concentration will generally be in the range of about 0.05 M/l. to 0.26 M/l.

In a particularly suitable embodiment of the invention the total phosphate content of the etching solution is above 0.2 M/l., but more preferably not above 0.3 M/l. (e.g. about 0.22 M/l.), the pH is about 1.4 to 1.6 and the titratable hydrogen ion concentration is significantly less than the total phosphate ion concentration (e.g. the difference between the total phosphate ion concentration and the titratable hydrogen ion concentration is more than 0.01 M/l. and preferably at least 0.03 M/l.), the titratable hydrogen ion concentration most preferably being in the range of 0.12 to 0.18. It is also preferred that the ammonium persulfate concentration be about 0.6 to 1.1 M/l. and that the dissolved copper concentration be not above about 0.4 M/l.

To provide the phosphate ion it is preferred to include a compound containing orthophosphate ion, such as an 3 orthophosphate salt and/r phosphoric acid. The phosphate salt is preferably one which does not interfere with the etching or reactivation process. Copper phosphate, ammonium dihydrogen phosphate or diammonium hydrosignificant hydrolysis of persulfate occurs, e.g. an amount of hydrolysis equivalent to the formation of well over 0.1 mol of bisulfate (e.g. over 0.2 mol, 0.3 mol, 0.5 mol or 1 mol or more bisulfate) per liter of solution gen phosphate are preferred compounds, because these 5 during the period of operation. For instance, in a typical salts do not introduce foreign cations to the etching soluclosed system operating in accordance with the prior art tion. The copper phosphate can be prepared in the etchteachings the amount of hydrolysis was found to be ing solution by adding phosphoric acid and a basic copper equivalent to the formation of about 0.03 mol per liter, compound such as basic copper carbonate. or higher, of bisulfate per day. In a test of the system The process is preferably operated continuously. For inof the present invention the total hydrolysis was found stance the solution from the etching tank or other etching to be equivalent to formation of 0.2 mol per liter of bizone, where it is added at an elevated temperature, is fed sulfate over a 16 day period, which is only about 0.013 to a cooled crystallizing unit maintained at a lower temmol per liter of bisulfate per day. perature such that ammonium sulfate values and copper The following example is given to illustrate this inven- Sulfate values precipitate the resulting Solution is tion further. In this application all proportions are by then recycled to the etching zone. Probably because of weight unles otherwise indicated. gradual decomposition of ammonium persulfate to form ammonium bisulfate, the pH tends to drift, and a basic EXAMPLE I compound (preferably a basic copper compound) is pref' continuous etchin and re eneration recess is embly used to maintain the PH m the range descnbe.d 20 cafr ied out, in which tlfe etching solution is circulated abqve; thus copper Carbonate CPCQYCMOEDZ) continuously through (a) a spray-etching zone (in which .sohd P may added the sqluuon i the crystaihzthe solution, at 43 C., is used for the etching of copper), mg unit, so that it reacts with an d1SS0l V6S 1n the solution. (b) a crystallization zone at about 0 and (c) a feed The ammonium persulfate concentration of the solution tank at about C. During the Process metallic y 3 mamtamffd wlthm Jgi descnbefi above copper is fed at the etching zone and is dissolved, solid 32? 2; gg ggg i a ps of amniomum ammonium persulfate is added at the feed tank to refortify a S 0 so ammomum p-ersu e the etching solution, the copper sulfate and the ammonium (in water) may be added to a stirred feed tank in whlch If t 1 u d fro the etchin are removed from the solution is held after it leaves the crystallizer and E a e e m It u before it enters the etching zone. Losses of etching solu t e gf lllzalon Zone todresu m} e o approxl tion due to evaporation and drag-out reduce the phosf v6 copper an ammomum pets a 6 60mm phate ion concentration; this may be conveniently retratlons' plenished by the addition of H PO (e.g. aqueous 85% H PO added along with the basic copper compound) or by the addition of other sources of phosphate ion, such Etching Crystallization Feed as ammonium phosphate. The additions may be made zoneM/l' zone tankM/l' to either the crystallizing unit or the feed tank. The etch- D ss ed er ing solution generally also contains an activator as de- Ammmmm persulfate scribed in the aforementioned Caropreso, Radimer and Hogya patent (e.g. HgCl in amount such that the mer- 40 cury concentration in the solution is about 2 to 8 ppm, Basic copper carbonate is added at the crystallization zone more preferably about 3 to5 p.p.m.). as needed to control the pH in the desired range. The The temperature at which the etching is carried out etch solution has a mercuric ion concentration of about 5 is usually an elevated temperature, e.g. in the range of p.p.m. (e.g. mercuric chloride salt) and a total phosphate about 30 to 55 C. preferably about 35 to 38 to 50 C. concentration of about 0.22 M/l., which resulted from The etching solution may be applied in a manner well the inclusion of a mixture of copper phosphate (about known in the art, e.g. by spraying it onto the work or 0.05 M/l.) and phosphoric acid (about 0.17 M/l.), to dipping the work into a batch of etching solution, etc. produce an initial pH of the etching solution at 1.2-1.6. Such techniques are described in more detail in the afore- A dilute aqueous solution containing about half of the mentioned Caropreso, Radimer and Hogya patent, which mercuric ion and phosphate compounds, mentioned also describes suitable cooling procedures and temperaabove, is also added at the feed tank as needed, to comtures for the removal of ammonium sulfate and copper pensate for losses due to evaporation or drag out. During sulfate values. The feed tank in which the etching solua 16 day period of operation, with the process operating tion is stored, on its way to the etching zone, is usually continuously during working hours on 9 /2 working days maintained at, say, about room temperature, (e.g. in (about 7 hours per day, 5 days per week), analyses, such the range of about 20 to 30 C.); the cooled solution as the following, are obtained for the etch solution:

Etching zone analysis, Feed tank analysis M/l.

Ammo- Total nium Dissolved Phos- Titratable Ammo- Timeatter start oirun persulcopper, phate, hydrogen HSOr nium Dissolved (days) fate, M/l. M/I. M/l. ion, M/l. M/l. pH persulfate copper from the crystallizing zone may be warmed to this temperature and the solution passing from the etching zone may be cooled simultaneously, by passing these solutions through a suitable heat exchanger.

It will be understood that the invention is particularly intended for use in closed systems in which the persulfate solution is repeatedly (e.g. continuously) reactivated and recycled and kept in the system over a relatively long period of time, (e.g. well over two weeks, usually more than a month, generally over 6 months) during which Over the 16 day period, an average of about 0.7 g. of basic copper carbonate is used (for maintaining the pH level) per liter of the etching solution, per day. This indicates that the etching solution, maintained within the concentration ranges described above has an ammonium bisulfate build-up rate of about 0.013 M/L/day which is about one half the rate of an unmodified system. Tin-lead solder resisted or tin (e.g. with bright finish) resisted, one ounce/ sq. ft., copper laminated, printed circuit test panels are periodically etched in the etching zone throughout the 16 day period. Throughout the 16 day period the etching results are excellent, for both bright tin-resisted copper circuit boards and solder-resisted copper circuit boards. The solder resist is a lead-tin solder (60% tin, 40% lead). Both the bright tin resist and the solder resist are formed by electroplating limited areas of a copper base to form the circuit pattern. After etching, the circuit boards are simply rinsed with tap water and air-dried.

EXAMPLE II Another continuous etching and regeneration process is carried out, similar to the one described above, except that the etching solution contains 0.15 M/l. of phosphate and the pH is maintained at 1.5 to 1.8. In this run, concentrated ammonium hydroxide solution is added at the crystallizing zone for maintaining the pH. Over a 17 day period, analysis indicates an ammonium bisulfate buildup rate of only 0.016 M/L/day for this etching solution.

The total phosphate concentration may be measured by the method of C. H. Perrin J. Ass. Ofi. Agri. Chem. (1958) 41, 758; (19 59) 42, 567.

The persulfate concentration may be measured by the following procedure: Pipet 1 ml. of etchant into a 250 ml. Erlenmeyer flask and add 50 ml. distilled water. Add about ml. of 25% H SO to the Erlenmeyer. Add exactly 25.0 ml. of 0.2 N Ferrous Ammonium Sulfate solution to the Erlenmeyer and allow to stand for one minute. Titrate with standard 0.5 N KMnO; to slight pink end point. Run a blank using the same procedure except that no sample is added to the flask. From the volumes (mls.) of KMnO used for titrating the sample and the blank, the persulfate content in mols per liter (M/l.) can be calculated. This concentration is equal to:

Volume of KMnO. Used for Blank Volume of KMnO. Used for Sample 2 XNormality of KMnOiX 1 (1) Allow adequate time for instrument to warm up.

(2) Adjust wave-length to 730 millimicrons.

(3) Set zero-point (without test tube in holder and with cover of sample holder closed).

(4) Fill a B & L /2" diameter colorimeter test tube at least half full of distilled water and insert to the sample holder. Close cover. Adjust light control until meter reads 100% transmittance.

(5) Repeat steps 3 and 4 until meter reads 100% upon insertion of test tube with water.

(6) Prepare sample:

Pipet ml. of etchant into 100 ml. volumetric flask and dilute to mark with distilled water. Shake vigirously. Sample is ready.

(7) Fill a second test tube about half full of sample to be measured and insert the test tube into the sample holder. Absorbance of the sample may now be read directly. The dissolved copper concentration of the etchant can be calculated as follows:

Dissolved Copper, M/l.=Absorbance x 0.98

The titratable hydrogen ion concentration can be determined as follows:

(1) Transfer precisely 1.0 ml. of sample solution into a 250 ml. Erlenmeyer flask.

(2) Add approximately 70 ml. distilled water and 5 drops of bromo cresol green indicator to the flask.

(3) Titrate the sample solution with standardized 0.1 N sodium hydroxide solution from greenish yellow to greenish blue. Read the burette and record the result.

The concentration of the titrable hydrogen ion can be calculated as follows:

Titrable Hydrogen Ion (M/l.)

=Volume of N aOHXNormality of NaOH The peroxymonosulfate (HSO concentration can be determined in the following manner: Pipet 25 ml. of etchant into a 500 m1. Erlenmeyer flask containing 100 ml. 1 N H and add six drops of 0.01 M OsO.; solution as a catalyst and an excess of 0.1 N HAsO (30 ml.). Allow the mixture to stir for about two minutes. Then titrate the unreacted HAsO with 0.1 N Ce (SO until a pale blue end point is reached. The equivalents of H80 is equal to the equivalents of HAsO added, less the equivalents of Ce(SO added. Thus the concentration of HSO in mols per liter (M/l.) is equal to:

[ (Volume of HAsO Normality of HAS02) (Volume of Ce (S0 Normality of Ce(SO /50 The etching solutions used in the preferred embodiments of the invention consist essentially of aqueous solutions containing ammonium ions, hydrogen ions, copper ions, persulfate (including both peroxydisulfate and peroxymonosulfate) ions, phosphate ions, sulfate ions, bisulfate ions (in some cases), together with very small concentrations (previously described) of mercuric ions with the accompanying chloride ions (from HgCl Since ordinary tap water is conveniently employed for the preparation of the etching solutions used in the preferred process, other ions will also be present, usually in very small amounts.

Typical printed circuit boards are produced by applying a layer of copper to a synthetic resin base (as by laminating coper foil to a stiff sheet of glass fiber-reinforced thermoset epoxy resin); applying an organic resist over the surface of the copper except in the areas which are to constitute the circuit; electrodepositing tin or solder (generally containing over 50% of tin) onto those areas not covered by the organic resist; removing the organic resist; and (in the step to which the present invention relates) etching away all the exposed copper not covered by the tin or solder. Further details are given in Hogya and Tillis US. Pat. 3,476,624 of Nov. 4, 1969 which also describes the addition of H PO to an ammonium persulfate etching solution.

It is understood that the foregoing detailed description is given merely by way of illustration and that variations may be made therein without departing from the spirit of the invention. The Abstract given above is merely for the convenience of technical searchers and is not to be given any weight with respect to the scope of the invention.

We claim:

1. Process for etching copper with an aqueous ammonium persulfate solution having the ammonium persulfate concentration of about .4 to 1.3 M/ 1., in which the etching solution is repeatedly reactivated over an extended period of time by repeated removal of metal sulfate values and ammonium sulfate values and inw hich substantial hydrolysis of the persulfate occurs during said extended period, wherein the improvement comprises maintaining phosphate ions in the etching solution during said period at a total phosphate ion concentration of 0.1 to 0.35 M./l. and the etching solution at a pH of 1.2 to 1.8.

2. Process as in claim 1 in which said time period is at least two weeks.

3. Process as in claim 1 in which the ammonium persulfate concentration of the etching solution is maintained at about 0.4 to 1.3 M/l. and the concentration of dissolved copper is no more than about 0.8 M/l., the titratable hydrogen ion concentration being about 0.05 to 0.26 M/l.

4. Process as in claim 3 in which the total phosphate ion concentration is maintained at a level of at least about 0.2 M/l. and the titratable hydrogen ion concentration is maintained at at least 0.01 M/l. below said total phosphate ion concentration.

5. Process as in claim 4 in which said titratable hydro- 7. Process as in claim 6 in which said sulfate values are gen ion concentration is maintained at at least 0.03 M/l. removed by cooling to about 0 to 20 C. below said total phosphate ion concentration.

6. Process as in claim 5 in which said total phosphate References Cited ion concentration iS H011 above 0.3 M/l., said titratable' 5 UNITED STATES PATENTS hydrogen ion concentration is maintained at 0.12 to 0.18 M/L, the pH is maintained at about 1.4 to 1.6, the am- 3,399,090 8/1968 15619 monium persulfate concentration is about 0.6 to 1.1 M/l. and the dissolved copper ion concentration is up to about JACOB STEINBERG Pnmary Exammer 0.4 M/1., and said solution contains about 3 to 5 p.p.m. 10 U S C] XR of mercury, the temperature of etching being about 30 to 252 79 2 

